1. Field of the Invention
The invention concerns a magnetic resonance system and a method and a non-transitory computer-readable data storage medium encoded with programming instructions to generate a pulse sequence for operating a magnetic resonance system.
2. Description of the Prior Art
Magnetic resonance is a known modality with which images of the inside of an examination subject can be generated (in the following the abbreviation MR stands for “magnetic resonance”). The dependency of the precession frequencies (Larmor frequencies) of excited spins on the magnetic field strength of the prevailing magnetic field of the magnetic resonance system is thereby used for spatial resolution. The prevailing magnetic field is composed of the basic magnetic field of the magnet unit of the magnetic resonance system and applied gradient magnetic fields. Typical methods for reconstruction of image data sets from magnetic resonance signals require a homogeneous basic magnetic field and strictly linear gradient magnetic fields.
Due to the dependency of the Larmor frequencies on the prevailing magnetic field, geometric distortions along the frequency coding direction (readout direction) result in the image data sets acquired from the magnetic resonance signals in the case of inhomogeneities of the basic magnetic field. The distortions are proportional to local deviations of the basic magnetic field and inversely proportional to the strength of the frequency coding gradient.
Given nonlinearities of the gradient fields, the distortions are situated both in the tomographical image plane and perpendicular to this given slice excitations with a selection gradient. In practice, such inhomogeneities of the basic magnetic field and nonlinearities of gradient fields cannot be entirely avoided. The deviations of the basic magnetic field—thus the inhomogeneity—should nevertheless be smaller than 3 ppm (“parts per million”) within a measurement volume of a magnetic resonance apparatus.
MR examinations are also conducted on patients with metallic implants if this is allowed by the implant manufacturer. However, depending on the material, size and possibly shape, such implants generate significant image distortions due to strong susceptibility effects since they in particular significantly disrupt the homogeneity of the basic magnetic field, which can lead to the distortions described above.
The distortions of the applied magnetic field that are caused by the implants can corrupt the MR examination to the degree that, for example, MR measurements which require a spectrally selective excitation (likewise controlled via the Larmor frequencies of the participating substances) can no longer be implemented due to the more severe development of artifacts. One prominent example of such spectrally selective MR techniques is spectral fat saturation. The fat saturation serves to make it possible to differentiate a fat signal (which otherwise appears bright in the generated image data) and, for example, signals of inflamed tissue or, respectively, fluid accumulations.
For MR examinations in the environment of implants, it is most often attempted to suppress fat signals in a different manner, for example via what are known as “inversion recovery techniques”, in particular STIR (“short tau inversion recovery”). Apart from the geometric distortions mentioned above, however, strong susceptibility artifacts in the environment of the implants also lead to significant artifacts (for example in the form of regions with incomplete fat suppression and/or in the form of regions with significant signal losses).
For MR examinations in the environment of implants, optimally high readout bandwidths (high receiver bandwidth, high resolution) and high bandwidths of the employed RF pulses (short RF pulses, thin slices) have previously been sought to reduce the image distortions. However, this does not always lead to the desired goal and often can also not be directly influenced by a user.